1. Field of the Invention
The present invention relates to cold cathode gaseous glow discharge display devices of the multi-digit or character indicator type and more particularly to features related to multiplexed operation of such indicators for reducing blanking requirements for streamer elimination.
2. Description of the Prior Art
Each digit or character position of cold cathode gaseous glow discharge multi-digit indicators to which the present invention relates typically comprises a plurality of cathodes arranged in a pattern such as a figure eight representative of each digit and a respective operatively associated anode. The plurality of digits of such tubes are generally contained in a common envelope with an ionizable gas such as neon. Such tubes generally are of the type disclosed in U.S. Pat. No. 3,675,065 issued July 4, 1972, "Planar Gas Discharge Indicator", by L. C. Warne and U.S. Pat. No. 3,675,066 issued July 4, 1972, "Planar Raised Cathode Alpha-Numeric Gas Discharge Indicator", by J. B. Armstrong et. al., both patents assigned to the assignee of the present invention. For multiplexed operation these tubes would be constructed with individual anodes associated respectively with each digit position.
In the case of multiplexed operation of such indicators, the corresponding cathodes of each digit are connected to each other and the cathode groups (comprising the figure-eight, for example) are selectively energized from a common decoder-driver while the anodes associated with adjacent digits are sequentially energized. A symbol is displayed at each digit position in accordance with the selective energization of the cathodes (which determine the symbol) at the instant of energization of each anode (which determines the digit position). The rate of sequential energization of the anodes is sufficiently rapid to preclude observable flicker of the display.
In order to achieve suitable operation without undesirable effects, it is necessary in prior art indicators to utilize a blanking interval between energizations of successive anodes, that is energization must be removed from a previously energized anode before excitation is applied to the next adjacent anode with a blanking interval in between. Alternatively the blanking interval may be applied with respect to the cathode excitation or with respect to both the anode and cathode excitations. In the absence of a blanking interval, a positive column "streamer" glow is likely to occur between cathodes of the previously energized digit position and the adjacent successively energized anode because the ionized gas in the space therebetween has not had sufficient time to deionize. Such streamers are undesirable as they tend to mar the clarity of the display presentation, and produce "glow" in undesired locations. Provision of a sufficiently long blanking interval between energizations of digit positions eliminates such streamers by permitting the gas between successively ionized digits to de-ionize before transferring the energization therebetween. If, however, the blanking interval is insufficient, streamers are still likely to occur. The use of a suitable blanking interval therefore eliminates the occurrence of streamers, but on the other hand has the undesirable effect of limiting the sequential excitation rate of the digits with the likelihood of attendant flicker or alternatively limiting the number of digit positions which can be multiplexed.
An alternative to blanking for streamer elimination is to provide interdigit partitions or other mechanical isolation means so that positive column streamers are prevented from occuring between electrodes of adjacent digits. This solution increases the manufacturing complexity and hence the cost of such indicators.